Eddy current techniques have been used for many years to inspect electrically conductive materials. Useful background information regarding the technology of nondestructive testing by eddy current techniques can be found in published articles such as: "Applications of a Phase Sensitive Eddy Current Instrument" by C. V. Dodd in Materials Evaluation, June 1964, pp. 260-262/272; "A Portable Phase-Sensitive Eddy Current Instrument" by C. V. Dodd in Materials Evaluation, March 1968, pp. 33-36; and "Thickness Measurements Using Eddy Current Techniques" by C. V. Dodd and W. A. Simpson Jr. in Materials Evaluation, May 1973, pp. 73-79/84.
Various eddy current techniques for inspecting electrically conductive materials have been described over the years in the patent literature. Representative patents, which indicate pertinent fields of search for investigating the prior art, include U.S. Pat. Nos. 2,939,073; 3,718,855; 4,006,405; 4,107,606; 4,230,987; and 4,351,031.